1. Field of the Invention
The present invention relates to an image signal processing apparatus, and particularly to signal processing means for producing an RGB signal by an RGB color ratio correlation. For example, the signal processing means is used for a solid-state imaging device such as a charge-coupled device (CCD) image sensor and CMOS image sensor, an image sensor built-in portable telephone, a digital camera, and a video camera.
2. Description of the Related Art
Recently, with finer pixels in an image sensor, a 2-μm order pixel is put into practical use, and a 1.75-μm pixel and a 1.4-μm pixel are being developed. In the pixel of not more than 2 μm, the incident light quantity is greatly decreased, causing the signal-to-noise ratio (S/N) to deteriorate. Additionally, in a conventional color camera, there is a problem in that image quality is caused to deteriorate by false color signal or color noise. There have been proposed various methods for suppressing the false color signal or reducing the noise (for example, see Jpn. Pat. Appln. KOKAI Publication Nos. 4-235472, 2002-10108, 2005-303731, 2001-245307, and 5-168029). However, there is proposed no effective countermeasure against the problem.
For example, in a single-plate color camera, an RGB color shift in which the false color is generated in an edge portion of a subject image or random noise of each of the RGB signals generates dot-shaped color noise to thereby cause the image quality to deteriorate.
There are two factors that generate the false color signal. First, beat noise is generated near a critical resolution of a sensor because of a mismatch between the pixel pitch and a high-resolution pattern of a subject, and a frequency of the input signal is folded back toward a low frequency to generate the false color signal. Second, refractive indexes of the RGB light beams differ from one another because of a difference in wavelength of the RGB light beams incident to an optical lens, thereby generating magnification chromatic aberration in which RGB images are shifted from one another in a surrounding area.
As shown in FIG. 18, the magnification chromatic aberration is particularly increased in an end portion of a pixel area. Furthermore, in order to correspond to the fine pixel, as shown in FIG. 19, when the F value is decreased while the spatial frequency is increased, on-axis chromatic aberration is increased even at the center of the screen because of a difference in decrease of the modulated transfer function (MTF). Therefore, the image quality deteriorates remarkably in the fine pixel.